The present invention relates to a room temperature vulcanizable polysiloxane composition, and more particularly, to a two-component room temperature vulcanizable polysiloxane composition containing terminal silicon-bonded hydroxy groups and having an extended work life.
Room temperature vulcanizable silicone rubber compositions are usually prepared either in a one-component form or a two-component form. Generally, in the two-component room temperature vulcanizable silicone rubber compositions, there is present a base silanol-stopped diorganopolysiloxane, optionally, filler and other additives which form a first component which is stored as such. As part of the composition, there is also formed a second component containing a catalyst which is usually a metal salt of a carboxylic acid, optionally, filler, and other additives. The second component is mixed and stored separately from the first component or base composition described above. One or both of the components described above must also have a cross-linking agent, such as, an alkyl silicate, or partial hydrolysis products of such alkyl silicates. When it is desirable to cure the composition to form a cured polysiloxane elastomer, the first component and second component are mixed, and the resulting mixture is allowed to cure at room temperature, and in certain cases, at elevated temperatures, to produce a silicone elastomer.
The catalyst component, that is, the second component, is maintained separate from the base component, that is, the first component, because if the catalyst component is mixed with the base component, then the materials begin to cure immediately to form a silicone elastomer, and they are generally completely cured within about 24 hours. Thus, for storage purposes, the catalyst component is maintained separately from the base component until it is desired to mix the components and apply them as desired to produce a silicone elastomer.
Examples of prior art two-component room temperature vulcanizable silicone compositions and improvements therein are found in U.S. Pat. No. 3,445,418, U.S. Pat. No. 3,696,090, U.S. Pat. No. 3,701,753, U.S. Pat. No. 3,702,835, U.S. Pat. No. 3,839,246, U.S. Pat. No. 3,847,848, U.S. Pat. No. 3,957,704, U.S. Pat. No. 4,124,560 and U.S. Pat. No. 4,131,588. The foregoing patents generally disclose that one of the components in the composition comprises a metal salt of a carboxylic acid as a catalyst to speed up the rate of reaction of the organopolysiloxane composition to form a silicone elastomer. It is generally disclosed in the prior art that the catalyst can be selected from the metal salts of carboxylic acids, and more specifically, that the catalyst can be selected from the metal salts of a monocarboxylic acid or a dicarboxylic acid where the metal ion varies from lead to manganese in the periodic table of the elements (U.S. Pat. No. 4,131,588). In other prior art references, for example, in U.S. Pat. No. 3,696,090, U.S. Pat. No. 3,701,753, U.S. Pat. No. 3,847,848, U.S. Pat. No. 3,888,815 and U.S. Pat. No. 3,957,704, it is disclosed that the metal ion of the metal salt of the carboxylic acid is one selected from the group consisting of lead, tin, zirconium, antimony, iron, cadmium, barium, calcium, titanium, bismuth and manganese. Generally, the carboxylic acids from which the salts of the metals are derived, are either soluble or insoluble in the organopolysiloxane and can be aliphatic or aromatic and include the naphthenate, hexoate, octoate, laurate and stearate.
In contrast to the platinum-catalyzed addition cured vinyl-stopped dimethylpolysiloxanes which are easily heat accelerable, it is well known that the condensation-cured silanol-stopped organopolysiloxanes do not possess appreciable heat accelerable cures. The addition-cured platinum-catalyzed systems can be formulated to give long "bath lives" at room temperature, and can then be cured rapidly at elevated temperatures. This feature has enabled the vinyl addition cured systems to enjoy substantial use in the potting or encapsulating of electronic components and in the coatings industry where a long work time with the catalyzed material is desirable to operate efficiently the dispensing equipment so that the material does not cure in the equipment, yet can be cured rapidly to achieve rapid production rates of the finished parts. Accordingly, it is desirable to formulate the silanol-stopped organopolysiloxane compositions catalyzed by the metallic salts of organic carboxylic acids to extend their "bath lives" or working lives so that they can be used in applications similar to those in which the addition cured vinyl-stopped organopolysiloxanes are used. Thus, it is desirable to extend the working life of the room temperature vulcanizable condensation cured organopolysiloxanes having terminal silicon-bonded hydroxy groups so that they can be used in the potting or encapsulating of electronic components and in the coatings industry where a long work time with the catalyzed material is desirable in order to operate efficiently the dispensing equipment so that the condensation cured organopolysiloxanes containing terminal silicon-bonded hydroxy groups do not cure in the equipment, yet cure rapidly in the presence of the metallic salts of organic carboxylic acid catalysts even at room temperatures.